Matrixing can be carried out on a stereo signal having a left hand and a right hand signal component L and R respectively, so as to obtain a mono signal M=a(L+R) and an difference signal A=a(L-R), where a.ltoreq.1, such as .sqroot.2/2.
Compression means for bit rate reducing a signal has been described in published European patent applications 457,390A1 (PHN 13.328) and 457,391A1 (PHN 13.329). Bit rate reducing the above signals M and A by such compression means results in these signals being contaminated with quantization noise. The aim of the compression means is to keep the quantization noise below the threshold of hearing. After transmission and receiving the quantized signals, the quantized signals are dequantized in the receiver, so as to obtain a replica of the signals M and A. The original stereo signal is retrieved by dematrixing the dequantized signals M and A. It has been found that the received stereo signal is sometimes affected by quantization noise which has become audible.
Matrixing is also present when transmitting a first main signal component (the left hand signal component L of a stereo signal), a second main signal component (the right hand signal component R) and an auxiliary component (a central signal component C), such that a first signal component L.sub.c is obtained which equals L+b.C and a second signal R.sub.c is obtained which equals R+b.C, and where the signals L.sub.c, R.sub.c and C are transmitted. Upon reception by a standard receiver not having a corresponding dematrixing circuit, the signal components L.sub.c and R.sub.c are used for supplying sound via two stereo loudspeakers to a listener. The listener is thus able to perceive the C transmitted component as well, even though he has a standard receiver.
More sophisticated matrixing schemes are discussed in J.A.E.S., Vol. 40, No. 5, May 1992, pp. 376-382.